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cis/trans isomers
#1 Posted : Monday, June 08, 2020 9:23:54 PM
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Hi there!

I was just wondering...(on page 63 of chemistry exam Krackers manual it states that) if Cis diastereomers have a dipole moment and therefor they have stronger IMFs and a higher boiling point, how does that also correspond to them not being able to form crystals as readily as trans molecules? what kind of IMFs are helpful or required for a compound to be able to crystalize easily?
#2 Posted : Wednesday, June 10, 2020 12:26:07 AM
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This is an interesting question, but is unlikely to be something you are expected to know on the MCAT.

The reality is that crystallization is complex - intermolecular forces keep molecules bound together and prevent them from escaping into the gas phase (thus increasing boiling point), however, intermolecular forces alone don't mean that a compound can easily crystallize (for example glass is an amorphous solid, despite its strong intermolecular forces, it doesn't have a regular repeating crystal lattice.

So the propensity of crystallization is not just dependent on a compounds intermolecular force, but also its geometry, how easily things can stack together and other factors.

In this case, cis compounds have a larger overall dipole moment than trans compounds, meaning they tend to boil at higher temperatures. However, they have a shape that is less "stackable" than a trans compound (imagine fitting multiple cis compounds vs multiple trans compounds together molecules like puzzle pieces). This results in less propensity of cis molecules to crystallize.

I find it unlikely that something this specific would be sprung upon you without warning, unless it was outlined in the passage.

I hope this helps :)
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